I believe it may effect it, but not by much, at least not on a BB. Also, I think it's at the bottom as it's the last place you will keep your hands positioned on the device.
So is this where *all* of the radio activity is? Phone, GPS, BT, etc.?
When I'm using my bb with one hand, I use my forefinger on the top of the phone and my pinkie finger on the bottom, with the two middle fingers on the back and thumb for the buttons. Probably not a big deal.
Also, cradle charger (pod) connects at bottom...could that affect reception/transmission?